1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery provided with a positive active material made of a composite oxide.
2. Description of Related Art
The importance of lithium-ion secondary batteries or other non-aqueous electrolyte secondary batteries as a power source mounted in a vehicle or a power source of a personal computer of a mobile terminal has increased. Particularly, a lithium-ion secondary battery capable of obtaining a high energy density at a low weight is preferably used as a high-output power source mounted in a vehicle. For such uses, a reduction in the size and weight of the battery is required, and an increase in the energy density of the battery has become an important technical object. In order to increase the energy density, increasing the operating voltage of a battery is effective.
Currently, as a positive active material included in a 4 V class lithium-ion secondary battery, a lithium manganese composite oxide (LiMn2O4) having a spinel structure or the like is used. When a positive active material having a higher potential is developed, a further increase in the energy of the lithium-ion secondary battery can be achieved. In order to achieve the increase in the density, a lithium nickel manganese composite oxide having a spinel structure in which a portion of manganese of LiMn2O4 is substituted with nickel is considered. Nickel contained in the nickel-containing composite oxide allows the lithium-ion secondary battery to operate in a voltage range of 4.5 V or higher. Therefore, the nickel-containing composite oxide is expected to be a positive active material capable of obtaining a high capacity and a high energy density.
In general, when the non-aqueous electrolyte secondary battery which uses the lithium nickel manganese composite oxide having a spinel structure as the positive active material undergoes repeated cycles, there is a possibility that Mn may elute from the positive active material. When Mn elutes from the positive active material, the deterioration of a negative active material or a non-aqueous electrolyte progresses due to the eluted Mn, and there is a possibility that the deterioration in the capacity thereof may occur.